In the game
Deus Ex, a "killphrase" is a phrase programmed into
UNATCO's mech-augmented agents, which kills them upon hearing it. The newer,
nano-augmented agents instead had "
killswitches" which, while having the advantage (from UNATCO's
point of view) of being remotely activated, for some reason take 24 hours to kill the agent. This is actually pretty important to the game's development; if
Bob Page or one of his
Majestic 12 cronies could have simply made Denton explode by saying "ooga booga" or something, he would have to avoid them at all costs instead of fighting several hundred of them. That would have made for a pretty boring game.
There are two times you can use a killphrase in the game. You learn Gunther Hermann's killphrase, "Laputan machine", if you have Jaime Reyes spy on UNATCO and then meet him at the cafe in Paris.
Anna Navarre has a killphrase as well. It's "Flatlander woman". You can get this after you escape the MJ12 facility if you didn't manage to kill her when you're running from UNATCO in Hell's Kitchen: one half of the phrase is in her computer, the other half is in Manderley's. I found this out after purposely getting captured before reaching Navarre; I wanted to see if Gunther might be persuaded to join your side if you didn't kill her. Of course, it turns out you have to kill her anyway; it's just a question of when.